Capacitors having a voltage-variable dielectric are being employed in tunable RF devices such as antenna tuners and tunable filters. The dielectric constant of a voltage-variable dielectric can be changed by varying a bias voltage across the capacitor, which results in providing a voltage controlled capacitance device.
Presently there are various kinds of voltage controlled or digitally tuned capacitor devices. A digitally tuned capacitor is an integrated circuit (IC) that may be based on one of several technologies. Such technologies may include Micro-Electro-Mechanical Systems (MEMS), Silicon-On-Insulator and Silicon-On-Sapphire (SOI/SOS) devices and Barium Strontium Titanate (BST) variable capacitive devices. MEMS, SOI/SOS and BST devices are presently available from a number of suppliers and vary in capacitance range, quality factor and resolution for use in different RF tuning applications.
MEMS devices tend to have the highest quality factor and are highly linear. MEMS devices are suitable for antenna aperture tuning, dynamic impedance matching, power amplifier load matching and adjustable filters.
SOI/SOS tuning devices are constructed of solid state field effect transistor (FET) switches built on insulated CMOS wafers and use metal-insulator-metal (MIM) capacitors arranged in binary-weighted values to achieve different capacitance values. SOI/SOS switches are highly linear and are well suited to low power applications where high voltages are not present.
BST variable capacitor devices are based on Barium Strontium Titanate and vary their capacitance when a bias voltage is applied to the device. The tuning capacity of BST devices is limited, in part, by the accuracy of a digital-to-analog converter (DAC) circuit that generates the bias voltage applied to the BST variable capacitor device. BST variable capacitor devices are employed in tunable RF devices, such as antenna tuners and tunable filters, because of the high sensitivity that their dielectric material has to an applied voltage for changing the dielectric constant of the variable capacitor device.